MRAM devices are a promising alternative to conventional dynamic semiconductor memories. MRAMs are nonvolatile memories, which, in contrast to conventional dynamic semiconductor memories, do not need a refresh process for information retention. MRAM memory cells are substantially formed of two magnetic layers with a nonmagnetic layer arranged in between the two magnetic layers. MRAMs are resistant to radiation, so that information retention is ensured even when radiation is incident.
An MRAM memory cell is based on ferromagnetic storage with the aid of the tunneling magnetoresistance (TMR) effect or the giant magnetoresistance (GMR) effect. A conventional MRAM cell is constructed of a layer stack comprising a soft-magnetic layer (storage layer), a tunnel oxide layer and a hard-magnetic layer (reference layer) arranged at the crossover point between bit and word lines. Magnetization of the reference layer is predefined, while the magnetization of the storage layer is adjustable by sending corresponding currents in different directions through the word line and the bit line. By controlling these currents, the magnetization of the storage layer can be set parallel or antiparallel with respect to the magnetization of the reference layer. In the case of a parallel magnetization of storage layer and reference layer, the electrical resistance in the stack direction of the layer stack (i.e., from top to bottom or vice versa) is less than in the case of an antiparallel magnetization of storage layer and reference layer. This electrical resistance dependent on the different magnetization directions of the two layers can be evaluated as logic state “0” or “1”.
The magnetization of the storage layer that is parallel or antiparallel with respect to the reference layer is enabled by a magnetic anisotropy of the storage layer, which defines a magnetic preferred direction. The expression “preferred direction” is conventionally used in the art, although “preferred axis” would be more correct since both directions along the axis are equally preferred. Despite this, the expression “preferred direction” is used herein.
The magnetic anisotropy may be provided by shape anisotropy. Thus, in the case of a magnetic layer having an elongated shape, the magnetic preferred direction corresponds to the geometrical longitudinal direction of the magnetic layer. Due to the requirement that the leakage field energy be as low as possible, in energetic terms, the magnetization is directed collinearly with respect to the preferred direction of the anisotropy. By applying an external magnetic field, the magnetization of the storage layer can be switched back and forth between the two energetically preferred positions, if the activation energy required to overcome the energetically unfavorable intermediate positions is provided by the external magnetic field. In practice, such a shaped anisotropy of memory cells is realized, for example, by magnetic layers that are elliptically shaped in terms of their spatial form.
In the case of rotationally symmetrical magnetic layers, by contrast, the magnetic anisotropy is obtained as an intrinsic material property because an “in-plane” shape anisotropy cannot be realized. The cause of intrinsic anisotropy is under debate, but electron diffraction data at amorphous layer materials permit a conclusion that anisotropic orientation of atomic pair axes in the direction of the magnetic field is a possible cause of the intrinsic anisotropy.
In conventional MRAM memory cells, magnetization of the storage layer is set parallel or antiparallel with respect to the magnetization of the reference layer since this makes it possible to obtain a maximum signal swing with regard to the change in resistance ΔR/R of the layer stack during magnetization reversal of the magnetization of the storage layer relative to the magnetization of the reference layer.
However, in a memory cell having a circular-disk-shaped configuration and a storage layer having weak intrinsic anisotropy, it is not possible to ensure that the magnetization of the storage layer is oriented collinearly with respect to the preferred direction. In general, a single cycle of the magnetization reversal of the storage layer establishes a remanent magnetization of the storage layer, in which case the magnetization is directed non-collinearly with respect to the preferred direction of the intrinsic anisotropy of the storage layer.
Therefore, it is desirable to provide an MRAM memory cell having a circular-disk-shaped geometry with a storage layer having only weak intrinsic magnetic anisotropy to avoid a reduced signal swing with regard to the change in resistance ΔR/R during magnetization reversal of the storage layer relative to the magnetization of the reference layer caused by a remanent magnetization of the storage layer.